AGRICULTURAL  EXPERIMENT  STATION 
UNIVERSITY  OF  CALIFORNIA 

BENJ.  IDE    WHEELER,    PRESIDENT 

COLLEGE    OF    AGRICULTURE  thomas  forsyth  hunt,  dean  and  director 

nr-DUCl     C  V  H.   E.  VAN    NOR  M  AN  ,  Vice-Director  and   Dean 

Btttl\LLLY  University  Farm  school 


CIRCULAR  No.  110. 

December,  1913. 

GREEN  MANURING  IN  CALIFORNIA. 

By  Chas.  B.  Lipman. 

By  green  manuring  is  meant  the  incorporation,  through  plowing  or 
similar  operations,  of  green  organic  matter  into  the  soil.  The  purpose 
of  it  is  to  maintain  or  increase  the  organic  matter  supply  of  the  soil, 
which,  for  reasons  below  given,  constitutes  one  of  the  most  important 
factors  in  soil  fertility. 

Any  green  crop  which  yields  a  large  tonnage  of  material  in  a  short 
time  may  be  used  to  add  organic  matter  to  the  soil.  There  are  some 
plants,  however,  which  possess  not  merely  the  power  of  yielding  large 
amounts  of  organic  matter,  but  also  the  power  of  gathering  from  the 
air  the  most  valuable,  commercially,  of  the  elements  of  plant  food — 
nitrogen.  These  plants — the  legumes — other  conditions  being  equal,  are 
therefore  to  be  preferred  for  green  manuring  purposes  to  the  non- 
legumes.  They  are  plants  which  belong  to  the  well  known  family  of 
which  peas,  beans,  vetches,  alfalfa,  the  clovers,  lupins,  and  many  others 
are  members.  More  specific  information  bearing  on  the  choice  of  green 
manure  crops  in  California  is  given  below. 

Nothing  more  emphatic  can  be  stated  in  support  of  green  manuring 
under  the  proper  conditions  than  a  description  of  the  role  played  by 
organic  matter  in  soils.     Such  a  description  follows : 

1.  By  its  binding  action  on  the  soil  particles  and  by  its  sponge-like 
absorptive  power,  organic  matter  makes  sandy  soils  more  retentive  of 
moisture.      (Improves  water  holding  power.) 

2.  By  its  light  yet  bulky  mass  it  prevents  the  extremely  fine  particles 
of  clay  soils  from  running  together  and  becoming  cemented.  (Improves 
essential  feature  of  aeration  in  heavy  soils.) 

3.  Organic  matter  contains  most  of  the  nitrogen  found  in  soils  and 
all  plants  except  the  legumes  are  absolutely  dependent  on  it  for  their 
supply  of  that  essential  element. 

4.  Organic  matter  is  the  source  of  energy  and  the  laboratory  in  which 
the  important  beneficial  soil  bacteria  manufacture  available  plant  food 
and  bring  about  other  changes  of  importance  to  plants. 

5.  Through  its  decomposition  by  soil  micro-organisms,  organic  matter 
acts  as  a  source  of  carbonic  acid  and  other  weak  acids  which  help  to 
dissolve  and  make  available  the  mineral  matter  necessary  to  the  growth 
of  plants. 

Green    Manure  Crops  for  California. 

Since  the  addition  of  nitrogen  to  California  soils  is  most  likely  to  be 
the  form  of  fertilization  most  needed,  it  is  obvious  from  what  has  been 
said  above  that  the  leguminous  cover  crop  deserves  first  consideration 
where  it  is  possible  to  obtain  even  a  fair  winter  growth,  for  through  it 


—  2  — 

we  can  obtain  not  only  the  organic  matter,  but  the  nitrogen  which  it 
obtains  from  the  air  besides.  In  the  practical  experience  of  fruit 
growers,  and  ranchers  generally,  the  common  vetch  (Vicia  saliva)  and 
the  Canada  field  pea  (Pisum  arvense)  have  given  the  best  results  as 
winter  cover  and  green  manure  crops  in  California.  The  bur  clover 
(Medicago  denticulata)  takes  the  next  place  to  the  two  mentioned.  In 
general,  the  vetch  does  best  south  of  the  Tehachapi  and  in  sheltered, 
and  isolated  valleys  in  other  places.  The  Canada  field  pea  is  to  be 
preferred  in  the  Sacramento  and  San  Joaquin  valleys  and  adjacent 
regions.  Exceptions  will  occur  to  this  as  to  other  rules,  but  the  state- 
ment will  serve  as  a  good  guide.  Bur  clover  will  do  well  all  over 
California,  but  it  does  not  produce  as  large  a  tonnage  as  the  vetch  nor 
the  field  peas.  Other  crops  which  may  be  used  when  prices  of  seed 
allow  are  fenugreek,  tangier  peas,  and  lentils. 

If  climatic  conditions  are  such  as  to  preclude  the  possibility  of  much 
growth  during  the  winter  when  ordinary  green  manure  crops  are  grown, 
either  the  vetch  or  the  Canada  field  peas  may  be  planted  together  with 
one  of  the  grains,  thus,  field  peas  and  oats,  or  vetch  and  barley,  or 
bur  clover  and  barley.  If  no  growth  at  all  is  made  by  the  legumes 
then  the  cereals  may  be  grown  alone,  provided  it  is  remembered  that 
they  do  not  add  nitrogen  to  the  store  already  in  the  soil  but  merely 
return  what  they  remove.  They  do,  however,  add  large  quantities  of 
organic  matter. 

Summer  Cover  Crops. 

The  cost  of  water  and  the  ease  with  which  it  can  be  obtained  are 
seldom  such  in  California  as  to  justify  the  growing  of  a  summer  cover 
crop  or  green  manure  crop.  Certainly,  the  experimental  trials  thus 
far  with  summer  cover  crops  have  not  given  results  which  justify  their 
use.  Where,  however,  water  is  cheap  and  can  be  obtained  in  large 
amounts  summer  cover  crops  will  pay  well.  This  is  especially  true  for 
soils  markedly  deficient  in  humus  and  nitrogen.  Under  such  circum- 
stances the  cow  pea,  velvet  bean,  or  soy  bean  will  give  very  good  results. 
These,  like  the  winter  growing  legumes  above  described  may  be  com- 
bined with  one  of  the  cereals.  The  Whippoorwill  variety  of  the  cow 
pea  gives  good  results  in  California.  About  50  pounds  of  seed  to  the 
acre  for  irrigated  orchards  and  other  irrigated  land  will  give  a  good 
stand.  Cow  peas  are  wrongly  named  and  are  really  beans.  They  arc, 
therefore,  summer  growing  crops  and  can  not  be  used  as  a  winter 
growing  green  manure  crop  like  the  vetch  and  peas. 

When   and    How  to   Plant  Green    Manure  Crops. 

Under  dry  farming  conditions  the  best  time  to  plant  a  green  manure 
crop  is  just  after  the  first  rains.  The  seed  should  be  drilled  in  and  about 
60  pounds  per  acre  of  the  vetch  seed  used  or  80  pounds  of  the  field  pea 
seed,  because  the  latter  is  a  larger  seed.  Of  the  bur  clover  seed  20 
pounds  per  acre  will  do.  If  vetch  is  combined  with  barley  or  rye  about 
20  pounds  of  the  cereal  with  about  45  pounds  of  the  vetch  will  be  a  good 
combination.  Similar  combinations  may  be  made  between  field  peas 
and  bur  clover  and  the  cereals. 

As  a  cover  crop  and  green  manure  crop  in  orchards  the  same  legumes 
above  recommended  either  alone  or  in  combination  with  the  grains  may 
be  employed.     In  orchards  which  are  irrigated  the  cover  crop  may  be 


—  3  — 

sown  in  August  or  late  in  July.  About  20  pounds  per  acre,  less,  of  the 
seed  is  needed  in  orchards  than  on  dry  farmed  land,  since  there  must 
be  considerable  waste  space  between  the  trees  owing  to  heavy  shade  in 
which  the  green  manure  crop  will  not  grow. 

Plowing    Under  Green    Manure  Crops. 

Plowing  under  of  green  manure  crops  should  be  done  as  late  as 
possible  consistent  with  maintaining  good  moisture  conditions  in  the 
soil.  This  will  necessarily  vary  under  different  conditions.  About  the 
second  week  in  March  will  be  a  good  time  for  dry  farmed  land  as  a 
general  rule.  If  the  rainfall  is  very  small,  plowing  under  should  be 
accomplished  earlier.  On  irrigated  lands,  it  may  often  be  feasible  to 
let  the  cover  crop  grow  until  about  three  weeks  prior  to  blossoming  time 
and  plowing  it  under  then. 

It  is  important  to  plow  the  green  material  as  well  as  any  other  form 
of  organic  matter  deeply  into  the  soil  and  cover  it  well.  This  is  par- 
ticularly true  for  loose,  porous  sandy  soil.  If  very  tall,  the  cover  crop 
may  first  be  dragged  down  with  heavy  chains  or  with  a  roller,  then 
cut  up  with  a  disc  run  both  ways  and  plowed  under.  Thorough  pack- 
ing and  cutting  up  with  a  disc  after  turning  under  is  also  important 
to  prevent  the  soil  from  drying  out. 

Precautions  to   be   Observed   in   Green   Manuring. 

1.  If  growing  vetch  or  field  peas  for  the  first  time,  take  no  chances 
on  possible  natural  inoculation.  (Directions  for  inexpensive  inocula- 
tion are  fully  given  in  Circular  No.  87  on  Alfalfa.) 

2.  When  water  is  scarce  do  not  grow  a  winter  cover  crop.  It  will  not 
only  rob  trees  of  water  in  the  case  of  an  orchard,  but  in  dry  farmed 
land  will  make  the  soil  too  dry  to  grow  a  profitable  crop  after  it. 

3.  In  addition  to  danger  in  cases  of  lack  of  water  from  robbing  the 
following  crop  of  needed  moisture,  it  must  be  added  that  when  the 
cover  crop  is  turned  under  the  soil  will  be  too  dry  to  decay  it.  There- 
fore, the  organic  matter  will  not  only  give  no  good  effects,  but  by  its 
bulky  nature  will  leave  the  soil  loose  and  porous  and  thus  help  it  to 
dry  and  lose  moisture  all  the  quicker. 


